It is known that the efficiency of a gas turbine engine is dependent upon the control of gas leakage between stages in both the compressor and turbine sections of the engine. Although the engine is typically designed and manufactured to very precise dimensional tolerances, it is necessary to provide a sufficient cold clearance between rotating elements and the surrounding assembly to accommodate the differential thermal growth between the parts as the engine assumes its normal operating temperature.
In order to maximize efficiency, it is common practice to utilize an abradable surface layer on a metallic substrate which would be placed such that the rotating member could penetrate into the abradable surface layer as a result of thermal and centrifugal expansion thereby providing what is essentially a zero clearance.
Typically the abradable surface layer is attached to a metallic substrate whose composition has certain inherent drawbacks. One problem is that the elements used in formulating the metal of the substrate have the potential of being unobtainable because of their scarcity or the instability of world markets. This is made even more critical when it is considered that the most common application for such seals is in the construction of jet engines, e.g. for military applications.
The commonly used metallic substrates also have a relatively high coefficient of thermal expansion compared to available abradable surface materials. Should this differential between the two materials be too great, delamination at the substrate/abradable surface layer interface may occur or the abradable surface layer may crack, either condition being deleterious to its function. This need for minimizing the differential in expansion coefficients of the two materials necessarily narrows the number of abradable material which may be successfully employed. This is especially true where the part is one of high precision and will be used in temperatures ranging from ambient to 3000.degree. F. (1648.degree. C.).
A further drawback to the metallic substrate is its susceptibility to oxidation. Since most abradable seals are used in the hostile environment of a jet engine, it is necessary that the seal not be adversely affected by oxidizing gases. Unfortunately, the metallic substrates that are best able to withstand such adverse conditions are special alloys containing critical elements and are relatively expensive. These alloys are also very heavy and weight is always a prime concern in the aviation industry, especially in times of increasing fuel costs.
Accordingly, what is needed in the art is an abradable seal with a substrate that has less dependency on critical elements, is lightweight and relatively non-oxidative and whose coefficient of thermal expansion allows for a greater variety of abradable surfaces thus facilitating seal design.